This invention relates to the preparation of aqueous slurries of fully liquefied, and at least partially solubilized, starch, having extremely high solids concentrations; i.e., substantially greater than 40%, by weight.
Starch is liquefied and/or solubilized for many reasons, such for the preparation of malto-dextrins and as the first step in saccharification processes to form starch syrups, dextrose, levulose, and the like. It is generally desirable to obtain such aqueous slurries having the highest dry substance concentration possible, but the maximum concentration obtainable (using conventional equipment and unmodified starch) is about 40% solids. This is because, of course, the starch undergoes gelatinization under conventional liquefaction processes with an attendant sharp increase in the viscosity (referred to as the viscosity peak), and a gelatinized unmodified starch paste of more than 40%, dry substance, is too viscous to be processed in conventional equipment. It is customary practice, therefore, to liquefy the starch at a solids content of not over 40%, and eventually employ a separate evaporation step to bring the solids concentration to the desired point. Evaporation, of course, requires special equipment and employs a good deal of energy.
It is known that starch can be solubilized with .alpha.-amylase under non-gelatinizing conditions and recent developments (see, for example, U.S. Pat. Nos. 3,922,199 to Hebeda et al, 3,922,200 to Walon et al, 3,922,198 to Kuske et al and 3,922,196 to Leach et al) show that starch can be effectively solubilized with bacterial .alpha.-amylase at temperatures higher then its "normal" gelatinization temperature without any observable gelatinization taking place.
These recently developed processes are limited to starch slurries of 40% or lower (unless special equipment is employed) and furthermore, when working at temperatures substantially above the normal gelatinization temperature (e.g. at 75.degree. C. for corn starch) it is necessary first to prepare the aqueous slurry of starch and enzyme at a lower temperature and then heat it to the final desired temperature, if gelatinization is to be avoided. If one adds starch and .alpha.-amylase directly to hot water, having a temperature above the normal gelatinization temperature of the starch, the starch promptly gelatinizes. Surprisingly, however, in the process of the instant invention, when the fresh granular starch is added to the aqueous liquefied starch slurry at temperatures of as high as 75.degree. C.-85.degree. C. or even above, no observable gelatinization, with attendant viscosity peak, occurs. Because of this surprising phenomenon it is possible to "build-up" the solids content of a fully liquefied starch slurry to the levels aforementioned.